Organic electroluminescent (EL) devices, or organic light-emitting diodes (OLEDs), are electronic devices that emit light in response to an applied potential. The structure of an OLED comprises, in sequence, an anode, an organic EL medium, and a cathode. The organic EL medium disposed between the anode and the cathode is commonly comprised of an organic hole-transporting layer (HTL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the ETL near the interface of HTL/ETL. Tang et al., “Organic electroluminescent diodes”, Applied Physics Letters, 51, 913 (1987), and commonly assigned U.S. Pat. No. 4,769,292, demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures have been disclosed. For example, there are three-layer OLEDs that contain an organic light-emitting layer (LEL) between the HTL and the ETL, such as that disclosed by Adachi et al., “Electroluminescence in Organic Films with Three-Layer Structure”, Japanese Journal of Applied Physics, 27, L269 (1988), and by Tang et al., “Electroluminescence of doped organic thin films”, Journal of Applied Physics, 65, 3610 (1989). The LEL commonly includes of a host material doped with a guest material wherein the layer structures are denoted as HTL/LEL/ETL. Further, there are other multilayer OLEDs that contain a hole-injecting layer (HIL), and/or an electron-injecting layer (EIL), and/or a hole-blocking layer, and/or an electron-blocking layer in the devices. These structures have further resulted in improved device performance.
Moreover, in order to further improve the performance of the OLEDs, a new kind of OLED structure called tandem OLED (or stacked OLED, or cascaded OLED), which is fabricated by stacking several individual OLEDs vertically and driven by a single power source, has also been proposed or fabricated by Tanaka et al. U.S. Pat. No. 6,107,734; Jones et al. U.S. Pat. No. 6,337,492; Kido et al. JP Patent Publication 2003045676A and U.S. Patent Publication 2003/0189401 A1; Liao et al. U.S. Pat. No. 6,717,358, U.S. Patent Application Publication 2003/0170491 A1, and commonly assigned U.S. patent application Ser. No. 10/437,195 filed May 13, 2003 entitled “Cascaded Organic Electroluminescent Device Having Connecting Units with n-Type and p-Type Organic Layers”, the disclosure of which is herein incorporated by reference. For example, Tanaka et al., U.S. Pat. No. 6,107,734, demonstrated a 3-EL-unit tandem OLED using In—Zn—O (IZO) films or Mg:Ag/IZO films as intermediate connectors and achieved a luminous efficiency of 10.1 cd/A from pure tris(8-hydroxyquinoline)aluminum emitting layers. Kido et al., “High Efficiency Organic EL Devices having Charge Generation Layers”, SID 03 Digest, 964 (2003), fabricated 3-EL-unit tandem OLEDs using In—Sn—O (ITO) films or V2O5 films as intermediate connectors and achieved a luminous efficiency of up to 48 cd/A from fluorescent dye doped emitting layers. Liao et al., “High-efficiency tandem organic light-emitting diodes”, Applied Physics Letters, 84, 167 (2004), demonstrated a 3-EL-unit tandem OLED using doped organic “p-n” junction layers as intermediate connectors and achieved a luminous efficiency of 136 cd/A from phosphorescent dye doped emitting layers.
Using organic “p-n” junction as an intermediate connector is effective for optical out-coupling and for easy fabrication of devices. However, it is also necessary to use inorganic intermediate connectors in tandem OLEDs as an alternative method. Therefore, searching for a stable inorganic intermediate connector is still needed.
Using IZO or ITO film as intermediate connector will have high lateral conductivity resulting in pixel crosstalk problems. Moreover, the fabrication of IZO and ITO films requires sputtering that can cause damage on the underlying organic layers. Although using V2O5 film as intermediate connector can limit pixel crosstalk, V2O5 has been classified as a highly toxic substance (see Aldrich Catalogue, for example) and is difficult to be thermally evaporated as well.